'Abusive' tax avoidance: what are the implications of HMRC's draft GAAR?

Following the recommendation by Graham Aaronson QC's study group that a general anti-abuse rule (GAAR) be enacted, HM Revenue & Customs and HM Treasury are consulting on a draft GAAR, to come into force on 1 April 2013. This raises questions of principle and practicality. What effects will the GAAR have on tax and trust advisers and the taxpayers who rely on their advice? Should trustees and individuals take steps in the time left before the GAAR's introduction to try to forestall its operation?